From a viewpoint of improving a yield in semiconductor fabrications, management of a surface condition in a bevel portion of a semiconductor wafer has recently been drawing attention. Since a number of materials are deposited on a semiconductor wafer repeatedly to form multilayer structures, unwanted materials and damage are left on a bevel portion which is not used for actual products. As a conventional way of transporting and holding a wafer, it has been typically practiced to bring a holding member (e.g., a robot hand) into contact with a rear surface of a wafer. However, there is an increasing demand for maintaining cleanliness of the rear surface, as microfabrication technique of a device advances and a wafer having a diameter of 300 mm becomes mainstream. Therefore, it has been more common in recent years to transfer and hold a wafer by bringing a holding member into contact with only an edge of the wafer. Under such a background, the unwanted materials and damage, left on the bevel portion, are likely to come off the wafer during various processes and can adhere to a surface of a device. Such materials, attached to the device, can affect a yield in products.
In this specification, a bevel portion is, as shown in FIG. 1, a portion B where a cross section of a periphery of a substrate has a curvature. A flat portion indicated by a symbol D in FIG. 1 is a region where devices are formed. A flat portion E extending outwardly from the device-formation region D by several millimeters is referred to as an edge portion, which is distinguished from the device-formation region D. That is, the bevel portion is a rounded section extending from the edge portion E to the rear surface of the substrate.
While a cross-sectional shape of the bevel portion varies widely depending on substrate manufacturers, the bevel portion generally includes, as shown in FIG. 1, a slope surface F adjacent to the edge portion E, an outermost vertical surface G, and a slope surface F adjacent to the rear surface. The slope surfaces F and the vertical surface G are connected to each other by smooth curved surfaces H.
A polishing apparatus using a polishing tape is known as a device for removing a film formed on the bevel portion of the substrate. This type of polishing apparatus has a press pad arranged at a rear side of the polishing tape and is configured to press a polishing surface of the polishing tape against the bevel portion of the substrate by the press pad to thereby polish the bevel portion.
FIG. 2 is a perspective view showing a conventional press pad. As shown in FIG. 2, a press pad 100 has a rectangular pressing surface 100a. This press pad 100 is disposed at the rear side of the polishing tape and presses the surface (i.e., the polishing surface) of the polishing tape against the bevel portion of the substrate with the pressing surface 100a. The press pad 100 is fabricated from rubber, sponge, or the like. For example, urethane rubber or silicon sponge with a hardness (e.g., 20 to 40 degrees) suitable for polishing is selected as a material of the press pad.
FIG. 3 is a plan view showing the press pad when pressing a wafer and when not pressing the wafer. As shown in FIG. 3, a wafer W to be polished has a disk shape. When the press pad presses the polishing tape (not shown) against the bevel portion of the wafer W, part of the pressing surface 100a of the press pad 100 is compressed as the result of contact with the wafer W. Consequently, a contact area between the polishing tape and the wafer W is increased and a polishing speed per unit time is improved.
FIG. 4 is a vertical cross-sectional view showing the press pad when pressing the polishing tape against the bevel portion of the wafer. As shown in FIG. 4, during polishing, the pressing surface 100a of the press pad 100 is deformed along the bevel portion. Consequently, polishing pressure becomes high at boundaries between a region where the polishing tape 200 is in contact with the wafer W and regions where the polishing tape 200 is separated from the wafer W as indicated by arrows in FIG. 4. Specifically, polishing pressure on the boundaries is high and polishing pressure on a central region located between these boundaries is low. This makes it difficult to polish the central region. In such a case, if polishing is performed for the purpose of completely removing a film or an organic substance (stain) on the central region, the boundaries are excessively polished.
In this case, use of a polishing tape having a thicker base member can increase the polishing pressure on the central region of the bevel portion. However, use of such a polishing tape with a thicker base member entails larger reels for supplying and recovering the polishing tape. Further, when the polishing tape with the thicker base member is used, a tension of the polishing tape can largely affect polishing of the substrate. As a result, polishing process becomes unstable.